Diffusion coating of metals



United States Patent 26,601 DIFFUSION COATING 0F METALS Richard L.Wachtell, Scarsdale, and Charles A. De Qulsto, Elmsford, N.Y., assignorsto Chromalloy Corporation. West Nyack, N.Y.

No Drawing. Original No. 3,073,015, dated Jan. l5 1963, Ser. No. 29,150,May 16, 1960. Application for reissue Jan. 12, 1965, Ser. No. 438,449

7 Claims. (Cl. 29-183.5)

Matter enclosed in heavy brackets appears in the original patent butforms no part of this reissue specification; matter printed in italicsindicates the additions made by reissue.

This invention relates to the diffusion of metals for the productionthereon of an outer strengthincreasing coating or layer of enhancedoxidation and thermal shock resistance at high temperatures and, moreparticularly, to the production of such a coating including aluminum andchromium and silicon by diffusion coating techniques on various articlesformed from the so-called superalloys, such as high nicke1 andhigh-cobalt alloys, for high temperature uses and applications.

Metallurgical developments in recent years have indicated, particularlyfor high temperature uses and applications, the desirability ofhigh-nickel and/or high-cobalt alloys (sometimes now referred to as*supperalloys) as having desirable physical properties for various hightemperature uses, such as, for example, the manufacture of rotor bladesand stator vanes for high temperature gas turbines where operation isdesired without failure of the part even during prolonged exposure totemperatures well above 1500 F. and, consequently, substantially abovethe temperature range at which failure or diminution of the strengthcharacteristics may be expected of even high temperature austenitic ornickel or chromium steels.

Although the nickel or cobalt or other superalloys ma exhibit, for avariety of uses, physical properties within a desirable range,particularly when subjected in use to ex tremely high temperatures, theoxidation resistance and/or erosion resistance of the surface of suchalloys, the resistance to thermal shock, and the strengthcharacteristics particularly when subjected for prolonged times to hightemperatures and substantial temperature variations, may be less thandesired for prolonged or severe use. The foregoing becomes particularlyemphatic when it is realized that it may be desired to use such hightemperature superalloys for various machine parts requiring somestrength even while operating at temperatures close to or within therange of the softening temperature or melting temperature of the metalitself and under conditions where thermally induced softening or warpingand/or damage induced by thermal shock cannot be tolerated for optimumlysuccessful operation of the machine.

If it is attempted to increase the oxidation and thermal shockresistance of the surface of such high temperature alloys by thediffusion coating thereinto of a metal such as chromium according toconventional diffusion coating techniques, some enhancement of theoxidation resistance and other properties may be experienced, but, ithas been found less than the optimum desired, particularly for extremelyhigh temperature uses. Alternatively, also according to conventionaltechniques, if it is attempted to enhance the desired surface or otherproperties of such alloys by applying thereto a surface coating ofaluminum, for example, as by an aluminum dip, etc., such a coating,however much it may initially enhance the surface characteristics, maybe found to fail at particularly high temperatures. Even with adiffusion coating of aluminum, difficulty may be experienced from thedeep diffusion of aluminum and some adverse effects of the diffusedmetal on other physical or metallurgical properties of the superalloypart.

Reissued Apr. 26, 1966 According to this invention, however, a diffusioncoating of a combination of silicon and chromium and aluminum isprovided for various metal articles, particularly high temperaturealloys of nickel and cobalt and the like, providing a diffusion coatingof a combination of substances into the surface of the base alloy forproviding oxidation and thermal shock and erosion resistance superior tothat obtained by the diffusion coating of any one or any two of thesethree substances, and enhancing the strength characteristics of thefinished and coated article, particularly at high temperatures, whilediminishing the disadvantages of thermal deformation and severe thermalshock.

One object of this invention is to provide, on metal articles of thecharacter described, a diffusion coating of enhanced resistance tooxidation and erosion and thermal shock, particularly at hightemperatures, and for imparting enhanced strength characteristics to thecoated article when subjected to high temperatures.

Another object of this invention is to provide a high temperature alloyarticle of the character described having on the surface thereof adiffusion coating of a combination of materials providing enhancedoxidation resistance for said article in use and enhanced strengthcharacteristics for said article particularly at high tcm peratures towhich it is subjected in use.

A further object of this invention is to provide a diffusion coatingcomprising silicon, chromium, and aluminum into the surface of hightemperature alloys such as those having a prepondercut nickel and/orcobalt base for enhanced oxidation resistance and strength increasingresults at the surface of said article particularly during hightemperature use thereof.

A still further object of this invention is to provide an article ofpredominantly nickel-containing and/or cobaltcontaining base alloys andhaving a diffusion coated substantially continuous surface layer or caseincluding silicon and chromium and aluminum for enhancing the oxidationresistance of said article, the thermal shock resistance thereof, andthe strength characteristics thereof particularly when used in hightemperature applications.

Other objects and advantages of this invention will be apparent from thefollowing description and the appended claims.

Before considering in further detail illustrative embodiments oftechniques and articles embodying and for practicing this invention, itmay be useful preliminarily to consider some aspects of the type ofsituations or applications in which the enhanced advantages of thisinvention may be particularly useful. Thus, the utility and advantagesof this invention are particularly to be noted in (but not limited to)the situations where various metal articles are use-d in applicationswhere a substantial mechanical strength and resistance to warping andthermal shock are desired, along with oxidation resistance, attemperatures ranging above 2000 F. for prolonged periods. As will beunderstood, such very high temperatures are generally considered abovethe range of temperatures where even high temperature austenitic steelscontaining substantial roportions of nickel or chromium lose strengtheven to the point of not being self-supporting. Hence, although theteachings of this invention are applicable to provide satisfactory andadvantageous diffusion coatings on such ferrous articles, theapplication whereof to such articles appears to present, as a practicalmatter, less emphatic advantages as compared to the utilization of thisinvention for enhancing the ultimate characteristics of articles formedfrom superalloys.

Similarly, from these superalloys (such as the high nickel and highcobalt alloys noted herein), there must be distinguished the so-calledrefractory metals such as molybdenum, etc., also conventionally utilizedfor high temperature metallurgical application, and, perhaps, withinoperational temperature ranges substantially in excess of 2000" F. Aswill be understood, however, the founding and metallurgicalcharacteristics of the so-called refractory metals and their alloys mayinherently impose or introduce some difficulties or limitations in thecasting, working, and/or fabrication of parts or articles from suchmetals, which difficulties are eliminated or mitigated or not found inthe handling or casting or machining of articles produced from the socalled superalloys as noted herein. Thus, as is well understood, thesituation may arise that the refractory metals are not available forproducing a particular part or article for high temperature use becauseof the inherent limitations in casting, fabricating, etc., regradless ofthe high temperature resistance of such metals, whereas thesuperalloys," while possessing adequate characteristics for casting orfabrication or producing of the article, may not exhibit the desiredhigh degree of creep strength, oxidation resistance, and resistance tothermal shock at the desired high temperature of operational use of thearticle.

In such cases, according to this invention, a base alloy which can bereadily cast or formed or fabricated into the desired article can beused for the production thereof and then the oxidation resistance,thermal shock resistance, and high temperature strength characteristicsof the formed article or part are satisfactorily increased by thediffusion coatings hereof to produce ultimately in the coated andfinished article the operational characteristics and properties desired.

As further illustrative of the foregoing, it was desired to cast a gasturbine inlet stator vane, having relatively thin walls and madeaccording to the so called precision casting or lost wax process, sothat the desired founding or casting process required high fluidity ofthe molten metal and other founding and metallurgical requirements inthe formation of the vane which were inconsistent with or difficult toaccomplish with various refractory or high temperature metals or alloyswhich might inherently be expected to give the desired thermalstability, etc., at the high temperatures for which the vane wasintended in use. By casting these vanes from a high-cobalt supcralloy,adequate founding and casting and forming techniques and results wereachieved. After casting of the vanes, a diffusion coating or case ofsilicon aluminum, and chromium was produced in the surface thereof inaccordance with this invention, and it was found that such techniquesnot only increased the oxidation resistance at high temperature towithin satisfactorily enhanced limits, but also substantially increasedthe overall strength of the vanes at high temperatures and made themless susceptible to warping and to damage by thermal shock in useall toan extent and at temperatures Way beyond the inherent properties orcharacteristics of the superalloy itself.

In another instance, involving the fabrication of a socalled beehive orsandwich panel structure consisting of flat plates or sheets withintermediate corrugated members for increased stiffness and strengthwith a minimal increase in weight, and material used for the fabricatedpanel was rolled sheets of a highcobalt superalloy of about 0.010"thick. With the application of the siliconaluminum-chromium diffusioncoating according to this invention to the sandwich structure afterfabrication, the compressive strength was increased to about twice theoriginal value, with actual additional strength in the fabricatedstructure stemming, apparently, from a bonding effect adjoining thejunctures between the flat plates and the corrugated members, inaddition to whatever enhanced metallurgical or surface properties areimparted to the part by alloying with the diffusion coated materials.

As will be seen from the foregoing, and from the additional explanationherein, the diffusion coatings embodying and for practicing thisinvention do more than provide enhanced oxidation resistance to thesurface of the base alloy being coated. They also appear to affect thefinal metallurgical properties of the finished article, as well asproviding certain mechanical added characteristics which may not bewholly attributable to a purely metallurgical function or mechanism. Forexample, alloyed articles coated in accordance with the presentinvention are found to be structurally sound and retain sufficientstrength or thermal shock resistance for the purposes to which they areput even at temperatures at which the lowest melting component of thebase alloy core of the article is actually at or above the temperatureof the softening point or melting point thereof, thus demonstrating aninstance where the diffusion coated case actually increases the thermalstability and high temperature strength of the article at temperatureranges substantially above that at which the uncoated base alloy wouldbe expected to function satisfactorily, and in addition to whateveroxidation resistance, etc., the diffusion coating may impart to thesurface of the coated article.

In accordance with the present invention, it has been discovered thatthe advantageous production of an outer casing or coating on,particularly, nickel-containing and/or cobalt-containing supcralloyarticles by diffusion into the surface thereof of a combination ofsilicon, metallic chromium, and metallic aluminum, produces a layer orcase of substantially enhanced resistance to oxidation and erosion, aswell as a casing of good and uniform adherence to the base alloyarticle, even during thermal shocks and deformations to which thearticle may be subjected in use, and a casing which actually enhances toa substantial extent the high temperature strength and resistancecharacteristics of the article, substantially superior to that providedby plating or dipping techniques, or even diffusion techniques of eithersilicon or chromium or aluminum alone or separately as a coating orcasing on the particular base alloys of the character to which thisinvention relates.

As will be understood, the characteristic of resistance to thermal shockis of particular importance with high temperature machine parts and,actually, is a somewhat related characteristic to the ultimate of hightemperature oxidation resistance, at least, in so far as the productionof an oxidation resistant coating on high temperature alloys subjectedto severe thermal shock conditions becomes important if the supposedlyoxidation resistant coating or outer casing is not maintained uniformlycontinuous in use and firmly adhered to the article during thermallyinduced deformation thereof so that fissures or other discontinuitiesmay occur in the oxidation resistant coating or case as a result ofthermal shock, which fissures, once having occurred, readily presenteasy access to the base metal of the article for axidation corrosionthereof. Such considerations, of course, become even more emphatic whenthe film continuity of the outer layer or casing of the diffusion coatedmaterials is also relied upon to impart some further strengthcharacteristics to the article at high temperatures of the order ofthose which would normally cause softening of the base alloy from whichthe part was originally cast or otherwise formed.

In the same connection, however, it should be noted that the same highdegree of oxidation resistance or strength enhancing characteristics orresistance to thermal shock may not, necessarily, inhcrc in all types ofhigh temperature resistant coatings which might be employed for theprotection or enhancement of superalloy parts, and that one or anotherof different techniques may be specifically employed for concentrationon whichever of the several optimum characteristics may be consideredparticularly desirable or ultimately important for a particular use orapplication.

As illustrative of the foregoing considerations, an inlet gas turbinestator vane may be subjected constantly to the extreme thermal shock ofbeing constantly subjected in use to a temperature differential of manyhundreds of degrees across merely the narrow width of the vane as whenone edge is subjected to hot turbine gases, while the other edge issubjected to gases at much nearer room temperature. With such a use, thecharacteristic of extreme thermal shock resistance of the diffusioncoating may be desirable to an extent which may outweigh characteristicsof increased strength and/or long term oxidation at elevatedtemperatures. In such an event, one might select a diffusion coating orsurface coating technique which produced or emphasized extraordinarilyhigh shock resistance, even at the expense of oxidation resistance orstrength characteristics. By contrast, one may cite the situation where,regardless of the thermal shock characteristics to which the part wassubjected, the particular installation or application might require acast superalloy part or article which would withstand prolonged contactwith high temperature oxidizing atmospheres necessitating a hightemperature oxidation resistance for hundreds of hours at, say, 2200 F.or more. In such a situation, then, one might select, for the diffusioncoating of a protective layer, even on the same superalloy, of amaterial or technique which emphasized the oxidation resistance, even ifthis characteristic were obtained at the expense of thermal shockresistance.

The foregoing is particularly mentioned because, as will be understood,there may be a variety of special treatments for superalloy castings orother parts for enhancing one or another of the ultimate characteristicsthereof, some of which may, indeed, accentuate certain ultimatecharacteristics at the expense of others, while surface coatings orcasings in accordance with this invention may, in other situations,provide enhanced characteristics for a particular application with,even, the same base alloy. For example, the copending application, S.N.807,025, filed April 17, 1959, relates to the diffusion coating ofhigh-nickel and high-cobalt superalloys with a coated casing comprisinga combination of chromium and aluminum for the purpose of enhancing theoxidation resistance and thermal shock resistance of metal articles asthere disclosed, and it has been found that such diffusion coating layeror case may be useful for many applications for which the differentsiIicon-chromium-aluminum diffusion coated layer or case in accordancewith this invention is also useful, but there still are importantdistinctions between the two techniques and the results thereof.

Quite apart from the strengthenhancing characteristics at extremely hightemperatures imparted to the coated su peralloy articles in accordancewith this invention, it has been found that, for some applications, thechromiumalurninum of the copending case actually does produce thermalshock resistance somewhat in excess of that produced by coatings inaccordance with the instant invention-while, at the same time, failingto produce oxidation resistant characteristics of the order of magnitudehere. Thus, considering a high temperature machine part where thespecifications require that the ultimate finished article Withstandwithout failure from thermal shock, for example, 600 cycles of beingrapidly heated from room temperature up to 2000 F. and thereafterrapidly quenched, an article coated in accordance with this inventionwithstands 800 of such testing cycles without failure, while articlescoated in accordance with the foregoing copending application mightwithstand 1000 such cycles. That is to say, whereas the coatings of thisinvention tremendously enhance the thermal shock resistance of thesuperalloys to an extent where parts made therefrom will pass the mostrigid conventional tests, still these coatings may not enhance thatparticular characteristic to the same extent as the chromium-alumiumcoatings of said copending application.

Nevertheless, when other characteristics of the finished part becomemore important for consideration, diffusion coatings or surface layersor cases embodying the siliconchromium-aluminum combination inaccordance with this invention are found, in other respects, to produceresults substantially in excess of the results produced by either theuntreated alloy part, the same part diffusion-coated with either siliconor chromium or aluminum alone, and/or the same part ditfusioncoated withany two of these three materials, including the parts of articles madein accordance with said copending application. For example, quite apartfrom the enhanced strength characteristics imparted to a metal articleembodying this invention and at temperatures close to or at thesoftening point or melting point of some components of the base alloy,coatings embodying this invention provide oxidation resistancesubstantially above prior coatings (including those of said copendingapplication) by as much as 20% or more. Thus, when oxidation resistanceis, for the particular use or application of the finished part, of moreultimate significance than thermal shock resistance, it has been foundthat various articles or machine parts processed in accordance with theteachings of this invention and including a diffusion coated layer ofsilicon and chromium and aluminum will withstand, under the usualtesting, oxidation resistance up to 2200 F. without failure, whereasother similar superalloy parts (even when treated in accordance with theaforementioned copending application) produce failures at, perhaps 2000F., from the standpoint of ultimate and prolonged oxidation resistancefor several hundred hours at such high temperatures.

As will be understood, in the comparative evaluation of the foregoing,it must be kept clearly in mind that it is possible, presumably, to usesome of the refractory metals or alloys thereof or so-called cermets forobtaining oxidation resistance and/or thermal shock resistance of theorders of magnitude mentioned above, but such possibility is completelyapart from consideration here, where the present invention relates to anappropriate trcatment of the castable and readily formable superalloysto impart to the article cast or formed therefrom the desired enhancedultimate characteristics required for a particular application. Also,any comparisons between one or another of the characteristics of thermalshock resistance or oxidation resistance or strength enhancingcharacteristics imparted to such superalloys and articles made therefromby a surface coating thereonparticularly at the temperature rangesnoted-must be evaluated in accordance with the particularcharacteristics desired and with regard to which one of the severalcharacteristics is particularly required for a particular application oruse. For example, although both the coatings in accordance with thisinvention and those in accordance with the above mentioned patentapplication produce different ultimate degrees of thermal shockresistance, both coatings are above the conventionally required demandsfor thermal shock resistance.

By contrast, however, considering, particularly, for example, a turbinblade or other part of a gas turbine engine of present design, despitethe fact that both coating techniques would produce, from the samesuperalloy, a metal article or part which would more than satisfy theparticular requirements of the part as to thermal shock resistance, thefailure of the coating of the copenJing application at 2000 F. would notsatisfy the requirements of the particular application or use whichdemanded an oxidation resistance which will withstand prolonged exposureto a temperature of 2200 F. without failure. As the various industriesrequiring such readily castable yet high temperature resistant materialshave developed today, particularly with regard to the extremes oftemperatures noted, a machine part or metal alloy article which willwithstand or pass an oxidation resistant test up to 2000 F. (but willnot withstand the test of 2200 F. as does an article treated inaccordance with this invention) is not merely not quite so good as anarticle coated or treated in accordance with this invention; it isactually unsatisfactory or inoperative in a situation rcquiring thehigher degree of oxidation resistance provided by the instant invention.

Thus, there may be superalloy articles having surface treatments thereofwhich permit them to exceed articles coated in accordance with thisinvention with respect to the one characteristic of thermal shockresistance, but which cannot withstand an oxidation resistant test to ahigh temperature level of articles in accordance with this invention.There may also be refractory metal or cermet articles which canwithstand oxidation resistance and/or thermal shock impact far in excessof articles treated in accordance with this invention; yet the veryrefractory metal from which they are produced precludes using suchmaterials in the formation of the particular articles to which thisinvention pertains.

Accordingly, this invention, with its production of a strengthincreasing, oxidation resistant, thermal shock resistant coating orsurface layer or case of silicon and chromium and aluminum, produces onreadily castable or formable or fabricatable articles of the superalloysan ultimate product meeting the specifications and tests andcharacteristics required for the high temperature machine parts or metalarticles desired; and these enhanced advantages are obtained, inaccordance with this invention, without regard to the fact thattechniques and processes and articles produced thereby and embodying orin accordance with this invention may not necessarily exceed all otherarticles with regard to all high temperature properties, as suggestedand will be understood by the foregoing comments.

Although the techniques in coatings embodying and for practicing thisinvention are satisfactorily applicable to a variety of various metalalloy articles, as noted above, they are particularly adapted for usewith base metal alloys containing a substantial or preponderantproportion of nickel or of cobalte.g., such superalloys as areparticularly formulated for high temperature use and having physicalproperties and a useful life as desired when subjected for prolongedduration to both very high temperatures and to the severe thermal shocksof rapid changes or great differentials of temperatures over wideranges.

Merely as illustrative of the types of high temperature alloys for whichthis invention is particularly adapted, one may note a commercialnickel-base alloy sold by Utica Metals Division of Kelsey-HayesCorporation under the designation of Udimet 500, a cobalt-base alloycommercially manufactured and sold by the Haynes-Stell te D vision ofUnion Carbide Corporation under the designation X-40, as well as anothercobalt-base alloy, also commercially manufactured and sold byHaynes-Stelhte, under the designation HS-25. Such alloys haveapproximately the following compositions (according to the respectivemanufacturers specifications) in welght percent.

High nickel alloy:

Iron (approx. Balance Also illustrative of the type of base materialwith which satisfactory results are achieved in accordance with thisinvention are the high temperature alloy steels, particularly theaustenitic steels and very low carbon steels and alloys havingsubstantial proportions of nickel and cobalt, although, as previouslymentioned, the enhanced advantages attributable to this invention maybe, for practical or economic or commercial reasons, less emphatic thanwith the nickel and cobalt superalloys noted.

Also, as further illustrative of the procedures and coatings aidtechniques embodying and for practicing this invention, it may be notedthat the articles of such high temperature alloys for which thisinvention is particularly adapted are satisfactorily produced or coatedby procedures including embedding the metal article to be coated in adry powder pack including an inert mineral filler material, a source ofthe silicon, chromium, and aluminum elements [to be diffusion coated],and a source of a vaporizable halogen material. As embedded in such apack (preferably contained within a metal container or retort the seamsof which are sealed by a fusible material such as a low melting silicateto prevent excessive escape of the diffusing materials during heatingand also excessive introduction of air into the pack during cooling),the articles are heated-all in known mannerto a substantial temperaturefor a number of hours to cause diffusion coating of the desiredmaterials into the base metal article surfaces, in conjunction with theelemental halogen reagent, etc.

Satisfactory results have been achieved according to this invention inso coating metal articles formed from any of the illustrative alloysmentioned above with the use of a coating pack comprising, asillustrative of this invention, approximately 60% alumina as the inertmineral filler, 30% chromium metal, 8% aluminum metal, 2% silicon (thelast mentioned as the three components to form the dilfusion coating),and up to 1% ammonium iodide as the vaporizable halogen source, theforegoing percentages being by Weight. With such a pack enclosed in acontainer in known manner for producing diffusion coatings for variousmaterials on various metallic alloys, satisfactory results have beenachieved according to this invention by heating the articles of the highnickel or cobalt content in such a pack for from four to twenty hours attemperatures of from about 1650 F. to 2100 F.

In connection with the foregoing ranges, it should be noted that, ifthinner coating layers or cases are desired (e.g., Where severe thermalshock and deformation of the articles are anticipated), the diffusioncoating is carried out, preferably, at the lower temperatures and/or forshorter times Within the foregoing ranges. Where thicker cases or coatedlayers may be desired (e.g., where oxidation and erosion resistance andstrength increasing are of more importance than resistance to possibledisruption of the coating layer or casing by thermal shock), thediffusion coating step is conducted at higher temperatures and/or forlonger times, thereby appropriately controlling the thickness of thediffused coating layer or casing produced in accordance with thisinvention.

As will be understood by men skilled in the art of diffusion coating ofmetals, the proportions of materials, and the materials themselves,suggested in the above mentioned pack may be varied over fairly Wideranges. Thus, the proportion of inert filler is not critical and,although alumina is a preferred filler material for such pack, otherinert fillers are satisfactory. Similarly, other halogen sources thanammonium iodide mentioned above give satisfactory results (e.g.,elemental iodine, ammonium fluoride, etc.) provided they are capable ofproducing a vaporized halogen at the temperature and under theconditions of operation, which halogen will act as a transfer agent orenergizer or aid for the introduction of the silicon and chromium andaluminum (or intermetallic complexes thereof) into the surface of themetal article being coated. Also as noted, the time and temperatureconditions of the diffusion coating step may, to some extent, be varied,depending upon the thickness of the coating or outer layer or casingdesired. Satisfactory results have been achieved with coatings of theorder of 0.001" to 0.002" thick, although even thinner coatings givesubstantially enhanced oxidation and other protection to the base alloyand provide the strength increasing characteristics therefor, and, forsome applications, thicker coatings may be desired.

Similarly, the relative proportioning of chromium to silicon to aluminumin the material of the diffusion coating or layer or case may varyconsiderably depending upon the final characteristics desired, it beingunderstood, of course, that equilibrium conditions may obtain in thecoating pack (with regard to the halides of the coating materials aswell as intermediate or intermetallic complexes thereof) over fairlywide ranges. It has been noted, for example, that, as the proportion ofaluminum in the diffusion coating is increased, a somewhat roughercoating surface is obtained (particularly, with alloys in which the ironcomponent thereof is larger than those mentioned above) and that thedepth or thickness of the coating (particularly the aluminum componentthereof) has some effect upon the ultimate metallurgical characteristicsof the coated article while, of course, the extent to which the siliconcomponent of the coating becomes alloyed with or included in the basemetal has its own metallurgical effect on the final characteristics ofthe finished article.

For example, it has been found that, generally speaking, increasing theproportion of aluminum in the final diffusion coating enhances theoxidation resistance of the finished coating, but also increases thebrittleness (or decreases the ductility) of the finished coating layeror case. Similarly it has been noted that the relative proportioning ofthe three components of the coated layer or case appears to be,generally, a function of the temperature at which the coating operationis conducted, at least for a given proportioning of the three componentsin the coating pack, [and with lower temperatures resulting in a lowerdiffusion of chromium into the surface being treated}.

In addition to the foregoing illustrative examples, it may be generallynoted that the proportions of the three constituents silicon andaluminum and chromium are satisfactorily provided in the coating packWithin approximately the following ranges: l%50% chromium, l%20%aluminum, and 0.5%15% silicon. Also it has been noted that, if the totalor aggregate of the siliconaluminum-chromium content of the pack exceedsapproximately 60% of the total pack composition, there may be a tendencyfor some undesired sintering of the pack com ponents during the heatingstep, and, at the other extreme, operable or satisfactory diffusion ofthe several elements desired is obtained in packs which have as littleas 5% of the total composition composed of the silicon and aluminum andchromium elements [to be coated].

As will be apparent, of course, within the range of avoiding undesiredside effects, some enhancement or efficiency may be produced byproviding as high as practicable a proportioning in the coating pack ofthe [three] elements desired to be diffused into the surface of thearticles embedded in the pack, as may be consistent, of course, with thesize and shape and volume and quantity of the embedded articles asrelated to the volume of the pack ingredients. As noted, also, the finalcomposition or the actual ratios in the finished coating will or may bevaried in accordance with the foregoing considerations, but, asillustrative, a pack in accordance with the foregoing example mentionedabove produces a coating where the constituents may be presentapproximately in the ratios of 90 aluminum to 5 chromium to 5 silicon byweight in the particular alloys mentioned and when coated in thepreferred temperature range of about l8002100 F.

In accordance with this invention, then, there is provided for thediffusion coating into high temperature alloys of a combination ofsilicon, and aluminum, [and chromium] under such circumstances where anouter coated layer or case is provided for increasing the oxidationresistance, strength, thermal shock resistance, and other properties ofa metal part or article fabricated or founded or formed from an initialmaterial or base alloy desirably susceptible to the fabrication orfounding or assembly techniques necessary or desired for the particularshape and formulation of the part or article. As noted above, and aswill be understood by men skilled in this art, the teachings of thisinvention are particularly applicable to high-nickel and high-cobaltsuperalloys, although the teachings of the invention produce enhancedthermal shock and oxidation resistance to other materials, particularlyprimarily ferrous articles. For example, satisfactorily enhanced resultsare produced by this invention on various types of stainless steelhaving high (eg. 35%) chromium and nickel contents and on variousaustenitic steels with or without substantial proportions of nickel andchromium and particularly on steels with low carbon contents;nevertheless, regarding such essentially ferrous base alloys ormaterials, the particular economical considerations from time to timemay indicate that a maximum oxidation resistance on such materials maybe obtained by means less expensive than the techniques disclosedherein, so that, although these techniques produce operative andenhanced characteristics on such other materials, economicconsiderations may indicate that the practical use of the techniquesdisclosed herein should be primarily relegated to the treatment of thehigh temperature superalloys for achieving the high degree of oxidationresistance and thermal shock resistance as noted herein.

Although the enhanced results of this invention are achieved with acoating pack comprising all three of the elements silicon, aluminum, andchromium, it now appears (from analyses of the coated productsaccomplished by well understood micro-probe techniques) that little, ifany, chromium is diffusion coated into the final coated article from thepack; but, rather, that the chromium constituent of the coating layer onthe coated article originates from the initial chromium content of thealloy base metal article (which, from the illustrative examples notedabove, is substantial and of the order of 19% or more). Whether or notthe foregoing be true or mcrcly a hypothesis, it nevertheless results,in accordance herewith, that the subjecting of such superalloy articlesto the coating steps and in a diflusion coating pack as describedproduces the enhanced results noted under the operating conditionsdisclosed above.

It is of course to be understood that the foregoing description isillustrative only and that numerous changes may be made in theconditions, proportions, and ingredients specifically disclosed withoutdeparting from the spirit of the invention as defined in the appendedclaims.

What is claimed is:

1. In a method for the production of a diffusion coating of thecharacter described on the surface of an homogeneous alloy base metalarticle having high temperature resistant characteristics and includinga preponderant proportion of a metal selected from the group consistingof nickel and cobalt, and a substantial chromium content, the stepswhich comprise embedding said alloy base metal article in a diffusioncoating pack including chromium metal and aluminum metal and silicon fordiffusion coating into the surface of said base alloy and a source ovaporizable halogen material as a carrier for said [chromium and]aluminum and silicon in said diffusion coating thereof, heating saidbase alloy in said pack effecting diffusion coating of said [chromiumand] aluminum and silicon together into the surface of said article.

2. In a method for the production of a diffusion ccating of thecharacter described in the surface of an homogeneous alloy base metalarticle having high temperature characteristics and including apreponderant proportion of a material from the group consisting ofnickel and cobalt, and a substantial chromium content, the steps whichcomprise embedding said alloy base metal article in a diffusion coatingpack including separate sources of chromium and aluminum and silicon fordiffusion coating into the surface of said base alloy and a source ofvaporizable halogen material as a carrier for said [chromium and]aluminum and silicon in said diffusion coating thereof, heating saidbase alloy in said pack to a temperature of the order of approximatelyat least 1650 F. effecting diffusion coating of said [chromium and]aluminum and silicon into the surface of said article to produce saiddiffusion coating thereon.

3. In a method for the production of a diffusion coating of thecharacter described for enhancing the corrosion resistance and thermalshock resistance of an homogeneous alloy metal article including apreponderant proportion of nicked and cobalt, and a substantial chromiumcontent, the steps which comprise embedding said article in a diffusioncoating pack including an inert mineral filler, separate sources ofelemental chromium and aluminum and silicon for diffusion coating intothe surface of said article, and a source of vaporizable halogenmaterial as a carrier for said chromium and aluminum and silicon in thediffusion coating thereof, heating said article and said pack togethereffecting diffusion coating of said chromium and aluminum and siliconinto the surface of said article to produce said oxidation resistant andthermal shock resistant coating thereon.

4. In a method for the production of a diffusion coating of thecharacter described for enhancing the corrosion resistance and thermalshock resistance of an homogeneous alloy metal article including apreponderant proporlion of nickel anti cobalt, and a substantialchromium content, the steps which comprise embedding said article in aditfusion coating pack including an inert mineral filler, separatesources of elemental chromium and aluminum and silicon for diffusioncoating into the surface of said article, and a source of vaporizablehalogen material as a carrier for said [chromium and] aluminum andsilicon in the diffusion coating thereof, heating said article and saidpack together effecting diffusion coating of said [chromium and]aluminum and silicon into the surface of said article to produce saidoxidation resistant and thermal shock resistant coating thereon, saidchromium and aluminum and silicon being present in said pack in amountsby weight of about 1592-5096 chromium, 1%20% aluminum and 0.5%15%silicon.

5. In a method for the production of a diffusion coating of thecharacter described for enhancing the corrosion resistance and thermalshock resistance of an homogeneous alloy metal article including apreponderant proportion of nickel and cobalt, and a substantial chromiumcontent, the steps which comprise embedding said article in a diffusioncoating pack including an inert mineral filler, separate sources ofelemental chromium and aluminum and silicon for diffusion coating intothe surface of said article, and a source of vaporizable halogenmaterial as a carrier for said [chromium and] aluminum and silicon inthe diffusion coating thereof, heating said article and said packtogether effecting diffusion coating of said [chromium and] aluminum andsilicon into the surface of said article to produce said oxidationresistant and thermal shock resistant coating thereon, said chromium andaluminum and silicon being present in said pack as approximately 5%60%thereof.

6. A metal article of the character described and susceptible to longexposure to oxidizing and thermal shock conditions which comprises anhomogeneous base metal alloy including a preponderant proportion of atleast one of the metals nickel and cobalt, and a substantial chromiumcontent, and which article is enclosed within an outer layer diffusioncoated case comprising chromium and aluminum and silicon.

[7. A metal article of the character described and susceptible to longexposure to oxidizing and thermal shock conditions which comprises anhomogeneous base metal alloy including a preponderant proportion of atleast one of the metals nickel and cobalt, and which article is enclosedwithin an outer layer diffusion coated case comprising chromium andaluminum and silicon, and in which said chromium and aluminum andsilicon are present in said outer layer diffusion coated case inapproximately the ratios by weight of aluminum to 5% chromium to 5%silicon] 8. A metal article of the character described and sitsceptibleto long exposure to oxidizing and thermal shock conditions and whichcomprises an homogeneous base metal alloy including a preponderantproportion of at least one of the metals nickel and cobalt, and asubstantial chromium content, and which article is enclosed Within anouter layer diffusion coated case comprising [chromium and] aluminum andsilicon, said outer layer diffusion coated case being at least about0.001"-0.002" thick over said article.

References Cited by the Examiner 2,612,442 9/1952 Goetzel. 2,801,1877/1957 Galmiche. 2,309,127 10/1957 Gibson. 2,811,466 10/ 1957 Samuel.2,955,958 10/1960 Brown.

FOREIGN PATENTS 3/1947 Great Britain. 2/1955 Great Britain.

MURRAY KATZ, Primary Examiner.

RICHARD D. NEVIUS, R. S. KENDALL,

Assistant Examiners.

